The present invention relates generally to ultracapacitors, and, more particularly to ultracapacitors having conducting polymers as the active material.
Electrochemical capacitors, also called supercapacitors or ultracapacitors, are energy storage devices which can store more energy than traditional capacitors and discharge this energy at higher rates than rechargeable batteries. In addition, the cycle life of electrochemical capacitors should far exceed that of a battery system. Ultracapacitors are attractive for potential applications in emerging technology areas that require electric power in the form of pulses. Examples of such applications include digital communication devices that require power pulses in the millisecond range, and traction power systems in an electric vehicle where the high power demand can last from seconds up to minutes. Battery performance and cycle life deteriorate severely with increasing power demand. A capacitor-battery combination has been proposed where the capacitor handles the peak power and the battery provides the sustained load between pulses. Such a hybrid power system can apparently improve the overall power performance and extend battery cycle life without increase in size or weight of the system.
An ultracapacitor is basically the same as a battery in terms of general design, the difference being that the nature of charge storage in the electrode active material is capacitive; i.e., the charge and discharge processes involve only the movement of electronic charge through the solid electronic phase and ionic movement through the solution phase.
Energy storage densities of ultracapacitors are much higher than those of conventional capacitors, but typically lower than those of advanced batteries. However, compared to batteries, higher power densities and longer cycle life have been either demonstrated or projected. These latter advantages of ultracapacitors over batteries are achievable because no rate-determining and life-limiting phase transformations take place at the electrode/electrolyte interface.
The dominant ultracapacitor technology has been based on double-layer type charging at high surface area carbon electrodes, where a capacitor is formed at the carbon/electrolyte interface by electronic charging of the carbon surface with counter-ions in the solution phase migrating to the carbon surface in order to counterbalance that charge. Conducting polymers also have been investigated for use in ultracapacitors. Higher energy densities can be achieved because charging occurs through the volume of the active polymer material rather than just at the outer surface. When a conducting polymer is being p-doped (positively charged), electrons leave the polymer backbone to generate an excess positive charge; anions migrate from the electrolyte solution into the polymer matrixes to counter the positive charge. In the case of n-doping of conducting polymers, the polymer backbone becomes negatively charged by the addition of electrons from the external circuit; cations enter the polymer matrixes from the electrolyte solution to balance the negative charge.
Another technology currently being pursued for the active material in ultracapacitors is based on noble metal oxides, predominantly ruthenium oxide. Charging in such active material has been reported to take place through the volume of the material and, as a result, the charge and energy densities observed are comparable with, or even slightly higher than, those obtained for conducting polymers. However, conducting polymers can generally be fabricated at significantly lower cost than noble metal oxides, with costs comparable to those of activated carbons. It is the combination of high energy density and low material cost that makes conducting polymers attractive active materials for ultracapacitors.
As disclosed in the U.S. Pat. No. 5,527,640 and described elsewhere, at least three different types of ultracapacitors can be constructed, using conducting polymer as electrode active material. A Type I capacitor is based on a symmetric configuration, with the same p-dopable conducting polymer active material on both electrodes of a cell. A Type II capacitor has an asymmetric configuration, with two different p-dopable active materials on the two electrodes. Relatively simple conducting polymers, such as polyaniline, polypyrrole and polythiophene, can be efficiently p-doped and can easily be synthesized from inexpensive commercially available monomers. However, the voltage window of a single cell device is limited in the range of 1 V to 1.5 V.
The most promising type is the Type III ultracapacitor that has a conducting polymer that can be charged both positively (p-doped) and negatively (n-doped). When a Type III capacitor is fully charged, one electrode is in a fully p-doped state and the other is in a fully n-doped state. When the capacitor is discharged, both electrodes will return to their undoped state. As a result, the cell voltage is increased to about 3 V and the full doping charge is released on discharge.
In addition to the increased energy density (since E=xc2xdCV2, where E is the stored energy, C is the capacitance of the device, and V is the voltage across the device), the Type III capacitor has two further advantages over Types I and II . Firstly, when a Type III capacitor is charged, both polymer electrodes are in a doped, highly conducting state and, therefore, the instantaneous power density at discharge is greater. In contrast, in a charged capacitor of Type I or II, one of the polymer electrode is in the undoped, semi-insulating state, resulting in a high impedance, which diminishes the instantaneous power density at discharge. Secondly, all the stored charge in a Type III capacitor is released at relatively high voltages. This is advantageous because charge delivered at a voltage that is too low may not be useful. Thus, the Type III configuration provides the best opportunity for an ultracapacitor based on conducting polymers to deliver the highest energy and power densities.
Previous studies have shown that poly(3-(phenyl)thiophene) derivatives substituted in the para-position, such as poly(3-(4-fluorophenyl)thiophene) (PFPT), are potentially active materials for the Type III ultracapacitor.
Various objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
In accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention is directed to a conducting polymer active material for use in an ultracapacitor. The conducting polymer active material is electropolymerized onto a carbon paper substrate from a mixed solution of a dimer of (3,3xe2x80x2 bithiophene) (BT) and a monomer that is selected from the group of thiophenes derived in the 3-position, having an aryl group attached to thiophene in the 3-position, or having aryl and alkly groups independently attached to thiophene in the 3 and 4 positions.